1. Field of the Invention
The present invention relates to the field of nonvolatile magnetic memories. In particular, the invention relates to a nonvolative magnetic memory which is transportable and does not incorporate or necessitate mechanical moving parts or moving magnetic domains or bubbles.
2. Description of the Prior Art
The first commercial random access nonvolatile magnetic memories were magnetic core systems in which circular magnetic ferrite cores were used as memory elements, were arranged in large 3-dimensional arrays, and were coupled to selected wire wraps for reading and writing. These first magnetic arrays were physically large, required large amounts of power and consequently were characterised by high heat dissipation, and were very expensive to fabricate.
Dynamic semiconductor memory arrays were then devised and are now predominantly used in small systems. However, a dynamic memory is volatile and because of this volatility cannot be transported. Practical nonvolatile memories thus remain to this date in the form of tape or magnetic disk. In both cases, tape and magnetic disks must be electromechanically read which introduces inherent unreliabilities, slowness in information transfer, and prevents such memory media from being truly random access. Furthermore, many of the nonvolatile magnetic memories such as tape and disk are not transportable or are transportable only under very carefully controlled conditions which are easily violated. Hard disks are, for example, hermetically sealed in their controllers and are totally nontransportable.
In addition to the foregoing memory systems, the prior art has also devised magnetic bubble memories wherein the presence or absence of a magnetic domain or bubble at a predetermined location within a supporting material is sensed and provides the operative memory element. However, in each of the bubble memories heretofore devised the presence or absence of the magnetic bubble could be sensed only if the magnetic bubble physically propagated in a predetermined manner through the supporting material, or if the stationary magnetic domain were sensed using special techniques such as sonic waves. For example, Myer, "Coercivity Control and Detection Signal Generating Pattern for Uniaxially Anisotropic Ferromagnetic Crystal Platelets", U.S. Pat. No. 3,971,038 discloses a nonvolatile memory system wherein a 2-dimensional lattice array of dots of magnetic susceptible material such as permalloy is in contact with a plane surface of uniaxially anisotropic ferromagnetic crystal platelets. The platelets have their major plane surface cut perpendicularly to the "easy" axis of magnetization of the crystal in order to be capable of sustaining movable, cylindrical, magnetic domains therein. Magnetic domains are moved between predetermined locations in the crystal by a magnetic field generated by electric drive signals in field generating conductor loops. By providing an array of dots, each of which is of a size having a maximum dimension which is a small fraction of the minimum diameter of the smallest bubble sustainable in the crystal, it is possible to control the coercivity of the crystal platelet, and thereby achieve some measure of stability of bubble position by the axis conductors and a greater temperature independence for the necessary biasing field. The lattice array which is used for coercivity control can also be used to sense a signal generated by the bubble as it is moved with respect to the array. As with virtually all bubble memories, movement of the bubble is used as the physical memory mechanism.
In another example shown by Kinsner et al, "Sonic Magnetic Domain Sensor", U.S. Pat. No. 4,094,003, a system is described wherein the presence or absence of a magnetic bubble in a magnetostrictive material layer is detected through the use of sonic waves. A sonic device launches sonic wave pulses which pass in the vicinity of a magnetrostrictive material layer. The sonic wave pulses stresses the magnetostrictive material and when the magnetostrictive material is magnetically influenced by a bubble, the stress changes or rotates its magnetization thereby inducing an electric signal in a conductor. When a bubble is not near the magnetostrictive material, no signal will be produced.
However, all bubble memories are susceptible to slow-switching rates, high temperature dependence, and low stability of the bubble position. Therefore, the nonvolatility of bubble memories is unreliable as a practical matter and the circuit systems and methodologies used for practical utilization of bubble memories are complex and often difficult to integrate into the more conventional circuitry of present computer and memory systems.
Therefore, what is needed is a simple, nonvolatile magnetic memory which is truly transportable, which provides true random access to the information stored therein, which does not necessitate the use of electromechanical components for reading or writing, and which is readily compatible with presently available computer systems.